The present invention relates to devices of the type known as vapor management valves (VMV) which are employed for controlling purge flow of fuel tank vapor from a storage canister to the intake manifold of an internal combustion engine. Such devices are employed in light motor vehicles where evaporation of tank fuel is prevented in the engine off condition by collection of the fuel vapors in a storage canister, typically of the type containing adsorbent granular charcoal.
Known VMVs provide an electrically operated bleed valve (EVR) for bleeding atmospheric air to a signal pressure chamber supplied with intake manifold vacuum for providing a vacuum control signal to one side of a pressure responsive diaphragm. The diaphragm operates a regulator valve member for controlling vapor flow between an inlet connected to the vapor storage canister and an outlet connected to the engine intake manifold. The diaphragm is preloaded by a spring to bias the diaphragm valve member closed preventing vapor flow to the engine manifold until a predetermined pressure differential is experienced by the diaphragm. An example of such a known VMV is that shown and described in U.S. Pat. No. 5,277,167.
Referring to FIG. 1, the known valve assembly indicated generally at 1 has an EVR indicated generally at 2 which controls atmospheric vent flow through a filter 5 and coil passage 3 to an outlet passage 4 which supplies air flow through an inlet passage 6 of a vacuum pressure signal chamber 8 which is supplied with engine manifold vacuum through a connector 10 and a single bleed orifice 12.
The pressure in chamber 8 is applied to one side of a pressure responsive diaphragm 14 which moves a regulator valve member 16 with respect to a valve seat 17 for controlling flow between vacuum connector 18 connected to the engine intake manifold and a fuel vapor purge inlet connector 20 connected to a fuel vapor canister 22 which is connected to a fuel tank 24. Diaphragm 14 is preloaded by a spring 26 to prevent opening of the valve 19 until a predetermined pressure differential exists across the diaphragm 14 in a manner well known in the art.
In the aforesaid known type of VMV, it has been found that the vacuum flow rate out of the vacuum pressure signal chamber increases with increasing engine manifold vacuum. At high engine manifold vacuum levels (reduced manifold absolute pressure), when a critical pressure ratio has been reached across the flow restricting orifice provided in the vacuum signal port, sonic flow choking or limiting occurs in the port thereby preventing further increases in flow with increasing engine manifold vacuum. For proper purge flow, it has been desired to provide a VMV having the properties that the atmospheric bleed flow increases with increasing vacuum (decreasing manifold absolute pressure) throughout the range of manifold pressures experienced during engine operation without the occurrence of sonic flow choking.
Where that engine throttle is closed suddenly or rapidly i.e., a condition referred to as "tip-out", the sudden large increase in manifold vacuum (decrease in manifold absolute pressure) causes sonic flow choking to occur at the vacuum restricting orifice; and, vacuum bleed flow no longer tracks engine manifold vacuum levels. Thus, it has long been desired to find a way or means of neutralizing the effects of "tip-out" on VMV control of fuel vapor flow to the engine intake manifold.